1. Field of the Invention
The present invention relates to magnetic recording disks, and more particularly, to single-layer and pseudo double-layer perpendicular magnetic recording disks with microsized domains.
2. Description of the Related Art
In longitudinal magnetic recording (LMR) applied to hard disk drives (HDDs), a major external data storage device of computers, the size of a data record domain in a magnetic disk has decreased with microstructure as the need for high-density data recording increases. However, this decrease in size makes the data record domains susceptible to removal by thermal energy generated by operation of the HDD which is more dominant than magnetostatic energy from the data record domain. This is referred to as the super paramagnetic effect. To overcome the super paramagnetic effect, the LMR technique has been replaced by a perpendicular magnetic recording (PMR) technique for HDD applications. The PMR technique uses a higher electrostatic energy and lower demagnetization energy compared to the LMR technique, so it is advantageous in high-density data recording. The high-density PMR technique also has enabled detection of a micro data domain in combination with advances in the manufacture of highly sensitive read heads.
In the PMR technique suitable for high-density magnetic recording, perpendicular magnetic anisotropy energy is exerted to orient the direction of magnetized domains perpendicular to the plane of a magnetic disk. Thus, head fields from a magnetic head should be induced to be perpendicular to the magnetic disk plane and thus parallel to the magnetized domains. To achieve this, a single-pole-type (SPT) perpendicular magnetic head is required. However, the SPT perpendicular magnetic head also generates a demagnetization field stronger than the perpendicular field of the magnetic head, so the perpendicular magnetic field induced by the SPT head is insufficient for recording, thus limiting use of the perpendicular magnetic recording technique in HDD applications.
The recent advances in magnetic recording technologies have enabled PMR with a ring-type magnetic head that has been used widely in LMP due to its ability to apply enhanced perpendicular magnetic fields for recording. Based on the PMR performed using the ring-type magnetic head, a single-layer PMR disk with a perpendicular magnetic recording/playback layer has been developed.
The schematic structure of a single-layer PMR disk is shown in FIG. 1. The single-layer PMR disk includes an underlayer 12 for promoting the perpendicular orientation of a perpendicular magnetic recording layer 13 formed over the underlayer 12, the perpendicular magnetic recording layer 13 having the perpendicular magnetic anisotropy energy to keep the perpendicular orientation of the data record domain, a protective layer 14 for protecting the perpendicular magnetic recording layer 13 from external impacts, and a lubricant layer 15.
The perpendicular magnetic recording layer 13 has the perpendicular magnetic anisotropy energy with a magnetic easy axis oriented perpendicular to the plane of the perpendicular magnetic recording layer 13 due to the underlayer 12. Therefore, perpendicular data recording can be achieved by perpendicular magnetic field components from a ring-type head. However, in the conventional single-layer PMR disk shown in FIG. 1, the perpendicular magnetic recording layer 13 having the perpendicular magnetic anisotropy energy has also a large demagnetization factor and thus strong demagnetization energy is induced in a direction opposite to the magnetic moment of the perpendicular magnetic recording layer 13, as expressed by formula (1) below:Kueff=Ku−2πNdMs2  (1) where Kueff is the effective perpendicular magnetic anisotropy energy, Ku is the perpendicular magnetic anisotropy energy, Nd is the demagnetization factor, Ms is the saturation magnetization, and 2πNdMs2 is the demagnetization energy.
Thus, the effective perpendicular magnetic anisotropy energy of the perpendicular magnetic recording layer 13 is abruptly decreased with unsatisfactory high-density recording properties, thereby limiting HDD applications of the perpendicular magnetic recording technique.
To overcome the effective perpendicular magnetic anisotropy energy reduction occurring in such a single-layer PMR disk, a pseudo double-layer PMR disk capable of reducing the demagnetization energy of its perpendicular magnetic recording layer has been developed.
In the pseudo double-layer PMR disk, as shown in FIG. 2, an intermediate soft magnetic layer 26 is deposited between a perpendicular orientation promoting underlayer 22 and a perpendicular magnetic recording layer 23 to allow formation of a closed magnetic circuit through the perpendicular magnetic recording layer 23 by perpendicular magnetic field components from a ring-type head. The closed magnetic circuit formed by the intermediate soft magnetic layer 26 reduces the demagnetization factor of the perpendicular magnetic recording layer 23 and its demagnetization energy, and thereby limits reduction in the effective perpendicular magnetic anisotropy energy.
FIG. 3 is a graph showing signal and noise level variations with respect to recording densities in kFRPI (kilo flux revolutions per inch) for the signal-layer PMR disk shown in FIG. 1 and the pseudo double-layer PMR disk with the intermediate soft magnetic layer shown in FIG. 2. In FIG. 3, -▪- and -□- represent the signal and noise levels, respectively, of the single-layer PMR disk, and -●- and -◯- represent the signal and noise levels, respectively, of the pseudo double-layer PMR disk.
The pseudo double-layer PMR disk shows a higher signal output than the single-layer PMR disk due to retention of the effective perpendicular magnetic anisotropy energy by the intermediate soft magnetic layer 26 that reduces the demagnetization energy by forming a closed magnetic circuit through the perpendicular magnetic recording layer 23. However, the intermediate soft magnetic layer 26 is also likely to cause a random orientation of neighboring magnetic fields and results in additional noise (jitter), so the pseudo double-layer PMR disk has a higher noise level than the single-layer PMR disk. Due to increases in both the signal and noise levels, the pseudo double-layer PMR disk has a signal-to-noise ratio which is too small for high-density recording. Therefore, there is a need to reduce a noise output level originating from the perpendicular magnetic recording layer 23 of the pseudo double-layer PMR disk to obtain a SNR large enough for high-density recording.
Reducing a noise level is also advantageous to the signal-layer PMR disk for improved SNR. For this reason, there have been continuing efforts to reduce a noise level amplified by the perpendicular magnetic recording layer itself in the single-layer and pseudo double-layer PMR disks for improved SNR.